When a hole is drilled in wood by any type of drill, a cylindrical hole is created. It sometimes occurs that the hole which has been drilled is undersized and re-drilling to a larger diameter is desired. Spiral wood bits rely upon a centering screw. Flat-bladed drill bits rely upon a center point. Neither of these common types of wood bits can be utilized for enlarging the previously drilled undersized hole, because they rely upon a centering structure on the bit to initially guide the bit.
Holes of larger size are usually bored with a flat-bladed drill bit. These are popularly called power bits, speedbor, spade bit, and paddle bit. When these types of drill bits are used, the center point of the bit is positioned on the center point of the hole to be bored. The bit is rotated and the bit is fed into the work. As the hole is started, the point maintains bit guidance. After a depth of about one-half the bit diameter has been reached, the bit blade sides alone are capable of maintaining further directional guidance of the bit. These flat bits have achieved wide acceptance due to several advantages over twist drills. Some of the advantages are wide availability, particularly in larger sizes. Flat-bladed drill bits can readily be resharpened with common hand files or grinding wheels. Furthermore, they are relatively inexpensive because of their simplicity. Flat-bladed drill bits are currently mass produced in sizes to drill holes up to one and one-half inch and 40 millimeters
As pointed out above, one shortcoming of the bit is its inability to enlarge existing holes due to the absence of material for engagement by the point of the bit. There is no material to act as centering structure for the second and subsequent bits. There is need for a device which permits flat-bladed drill bits to easily and accurately enlarge existing holes.